A functioning protein homeostasis network, which includes cellular chaperones and the ubiquitin-proteasome system, is critical to cellular viability and human health. This is evidenced by an ever-growing number of diseases associated with protein folding dysfunction. Viral infection also places a significant burden on the cellular proten homeostasis machinery, due to the large amount of a few structurally complex proteins during infection. We have shown previously that chemical inhibitors of the 90kDa heat-shock protein, Hsp90, potently block infection by numerous enteroviral species by inhibiting the processing of their capsid precursor protein, P1. Notably, in all of the viruses tested, resistant viral variantsdid not emerge, even after long-term passage suggesting that mutations that lead to folding of P1 independent of Hsp90 reduce fitness of the virus. This is supported by the observation that selection has significant effects on the composition of the viral population that persists after selection. The goal of this proposal is (i) to leverage new deep-sequencing technologies and population genetics to quantify the effect of Hsp90 inhibitors on viral population structure, and (ii) to biophysically characterize the mechanism of chaperone independence observed in the selected population. Our working hypothesis is that the inhibition of Hsp90 function will change the viral population structure and viral genome sequence to reflect the altered constraints on viral protein folding.